Point sampling on raster

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# Load the the multiband raster file ''clip_SA2_2017-03-28_pca.tif'', which should be available in the [[Course data|course data]]. It contains the first 5 component as a result of a pricipal components analysis of a Sentinel-2 scene.
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= First phase: systematic sampling =
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## Open {{mitem|text=Vector --> Research tools --> regular points}}. Set the {{button|text=Input extent Layer}} to ''Use layer/canvas extent''.  
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# Click {{mitem|text=Plugins --> Manage and Install Plugins}}.
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## Enter the number 600 (meters) to {{button|text=Point spacing}}.
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# Type in the search bar {{typed|text=Point sampling tool}}, click on the plugin name and then on {{button|text=Install plugin}}.
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## Click {{button|text=Use point spacing}} on.
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# Load the single band raster file ''C_ID.tif'' created by [[Convert vector to raster]].
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## Enter an output shapefile name ''systematic_grid.shp'' and path by clicking {{button|text=Browse}}.
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# Load the regular grid as vector point file ''systematic_grid100.shp''' created by [[Creating regular grid]].
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## Confirm with {{button|text=Run}}. The new layer is added to the Layer Panel .
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# Make sure that both layers are ticked in the [[TOC]].
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# Open the '''Point Sampling Tool''' clicking [[File:Qgis_psample_button.png]].
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# Select the layer containing sampling points.
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# Specify the output of the resulting vector file marking column with '''CTRL + left click'''.
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# Enter an output shapefile name ''systematic_grid100_cid.shp'' and path by clicking {{button|text=Browse}}.
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# Confirm with {{button|text=OK}}. The new layer is added to the Layer Panel .
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[[File:Qgis_point_sample.png|400px]]
  
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= Second phase: stratified random sampling with equal allocation =
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* In the search engine of the Processing Toolbox, type Random selection and select '''Random selection within subsets''' under Vector selection tools of QGIS.
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* Select the regular point grid (result of [[Creating regular grid]]) as the input layer.
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* Select the Column name containing the grouping class code as ID field.
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* Enter 10 as sample size for each category.
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* Click {{button|text=Run}} to execute the algorithm.
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* Mark the the regular point grid in the [[TOC]]. Right click, {{button|text=Save as...}}.
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* Format of the output vector file is '''ESRI Shapefile'''
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* Do not forget to tick '''Save only selected features'''.
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* Define directory and output file name.
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* Click {{button|text=Run}} to save the file.
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[[File:Qgis_vector_saveas2.png|400px]]
 
[[category:Working with Vector Data]]
 
[[category:Working with Vector Data]]

Latest revision as of 17:22, 27 November 2017

[edit] First phase: systematic sampling

  1. Click Plugins --> Manage and Install Plugins.
  2. Type in the search bar Point sampling tool, click on the plugin name and then on Install plugin.
  3. Load the single band raster file C_ID.tif created by Convert vector to raster.
  4. Load the regular grid as vector point file systematic_grid100.shp' created by Creating regular grid.
  5. Make sure that both layers are ticked in the TOC.
  6. Open the Point Sampling Tool clicking Qgis psample button.png.
  7. Select the layer containing sampling points.
  8. Specify the output of the resulting vector file marking column with CTRL + left click.
  9. Enter an output shapefile name systematic_grid100_cid.shp and path by clicking Browse.
  10. Confirm with OK. The new layer is added to the Layer Panel .

Qgis point sample.png

[edit] Second phase: stratified random sampling with equal allocation

  • In the search engine of the Processing Toolbox, type Random selection and select Random selection within subsets under Vector selection tools of QGIS.
  • Select the regular point grid (result of Creating regular grid) as the input layer.
  • Select the Column name containing the grouping class code as ID field.
  • Enter 10 as sample size for each category.
  • Click Run to execute the algorithm.
  • Mark the the regular point grid in the TOC. Right click, Save as....
  • Format of the output vector file is ESRI Shapefile
  • Do not forget to tick Save only selected features.
  • Define directory and output file name.
  • Click Run to save the file.

Qgis vector saveas2.png

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